Corner gets skewed by subdivision surface
This effect is a natural consequence of the Catmull-Clark algorithm, because the centers of adjacent faces,(and their sizes,) are used to calculate new edge-points.

If you're relying on Catmull-Clark to create your curve, and you want it to remain symmetrical about the corner, you will have not only to put in holding-loops for the corner itself, but at least one more loop to ensure the centers of the next faces along are also at an equal distance from the corner.